It has been known for over a decade that light emitting diodes and laser diodes make attractive light sources for pumping solid-state lasers. Laser diodes are capable of achieving high electrical to optical conversion efficiencies. The most efficient and powerful laser diodes are currently fabricated from GaAlAs and such lasers typically operate in the 780 to 880 nm wavelength range. Laser diodes of this type have been produced with an electrical to optical conversion efficiency in excess of 50%. Unfortunately, the beam from a laser diode is far from ideal for most conventional laser applications. At high powers, laser diodes generally operate in a large number of optical modes resulting in an output beam having a large divergence (typically 10.degree. by 40.degree.) and the output beam is generally of very poor coherence. However, by using one or more laser diodes as a light source for pumping a solid state laser, the light from the laser diodes may be converted into highly coherent light. Moreover, the emission wavelengths from laser diodes can be made to coincide very well with strong absorption bands of several lasing ions, such as neodymium, and so high optical to optical conversion efficiency can be achieved between the pumping light and the laser light.
In solid-state lasers pumped by laser diodes, two geometries are known for supplying the pumping light to the laser medium. In the end-pumped geometry, the pumping light is delivered to the laser medium along the axis of the optical cavity and through an end face of the laser medium. With this end-pumped geometry it is relatively easy to match the laser mode volume (volume in which the lasing process occurs) with the pumped volume and to achieve a relatively long absorption path for the pumping light. Thus, a relatively high optical to optical conversion efficiency may be achieved. However, the power that may be achieved in the end-pumped geometry is restricted by the requirement to focus the pumping light to a small spot. For example, in U.S. Pat. No. 4,710,940, there is described a laser having the end-pumped geometry in which an overall efficiency of 8% is achieved but with a continuous wave output power of only 80 mW.
In the side-pumped geometry, the pumping light passes through a side face of the laser medium in a direction generally perpendicular to the axis of the optical cavity. With the side-pumped geometry, the laser medium can be pumped with a multiplicity of one or two dimensional arrays of laser diodes and so this geometry does not suffer from the power restriction of the end-pumped geometry. In a typical arrangement using the side-pumped geometry, the absorption path for the pumped light is relatively short and the pumped volume is much larger than the laser mode volume in the laser medium; consequently, the optical to optical conversion efficiency is low. In "Laser Diode Side Pumping of Neodymium Laser Rods", Frank Hanson et al, Applied Optics, Volume 27, January 1988, there is described a laser using a side-pumped geometry in which a reflective coating is placed around part of the circumference of a laser rod. The reflective coating provides a double pass of the pumping light and thereby improves absorption and hence efficiency. However, this increase in efficiency is achieved at the expense of beam quality.
It is an object of this invention to provide a new or improved optically pumped laser in which the above mentioned problems are overcome or reduced.
According to this invention there is provided an optically pumped laser comprising an optical cavity having an optical axis, a laser member located in the optical cavity and formed at least partly from active laser material, means for providing pumping light and arranged so that the pumping light passes into the laser member through at least one side surface thereof, and means for deflecting the pumping light so that pumping light initially travelling from the pumping light providing means towards the laser member in a direction normal to the optical axis is deflected and caused to make at least one pass across the laser member in a direction having a substantial component along the optical axis.
By deflecting the pump light so that it makes at least one pass across the laser member in a direction having a substantial component along the optical axis a relatively long absorption path for the pumping light may be achieved. Also, in the laser of this invention, the pumping volume may be closely matched to that required for the laser mode volume. Consequently, a relatively high overall electrical to optical efficiency may be achieved in a laser embodying this invention.
Preferably, the deflecting means is arranged so that light initially travelling in a direction normal to the optical axis is caused to make at least two passes across the laser member in directions having substantial components along the optical axis.